It is well known that many polymers can be produced as powders in fluid bed reactors wherein the fluidization is provided by a circulating mixture of gases that includes the monomers. This technology is widely used commercially for polyolefins and polyolefin copolymers. One particularly useful arrangement of a fluid bed polyolefin process is disclosed in U.S. Pat. No. 4,882,400. Other examples of fluid bed polyolefin technology are demonstrated in the references that follow.
The active, growing powder in a fluidized bed polyolefin reactor is composed of a wide range of particle sizes. Thus, this powder is referred to as having a broad particle size distribution. Some of the reasons for this broad size distribution are the size range of the initial catalyst particles (or prepolymer particles) charged to the reactor, the difference in catalytic activity of each catalyst particle, the difference in residence time for each growing polymer particle, the agglomeration of polymer particles, and the spalling of polymer particles.
The size distribution of particles can be characterized by various physical measurements relating to the particle mass, physical dimensions, or specific surface area. Two commonly used methods of measurement, owing to ease and reproducibility, are mechanical sieve analysis and the light scattering behavior of a cloud of particles. The very small polymer particles are called fines. As used in the art, the term "fines" refers to some defined fraction of the polymer powder particles that are smaller than the average of the entire population of powder particles present in the fluid bed. Particularly small polymer particles, for example, smaller than 125 microns are considered fines.
In the fluid bed processes for the production of polyethylene and ethylene copolymers, high levels of polymer powder fines in the reactor pose significant and well known operating difficulties. Within the reactor, a higher level of fines often leads to increased agglomeration of the polymer powder. Outside the reactor, the fines may deposit in the recycle system and grow, fouling the piping, heat exchangers, compressors, and the reactor inlet gas distribution grid.
Inside the reactor, fines are a leading contributor to the formation of powder agglomerates. For various reasons, fines tend to segregate into certain poorly circulated and poorly cooled regions of the reactor. Exacerbating the segregation problem is the tendency of fines to have higher than average catalytic activity, thus tending to be hotter than the average particle. This higher catalytic activity in fines is due to higher concentrations of active catalyst components in the fines and due to short diffusion paths for monomers and for co-catalyst molecules.
One undesirable place at which fines accumulate is along the reactor vessel wall in the zone occupied by the main fluid bed. This accumulation is believed to occur because fines are more greatly affected by static forces due to their larger ratio of surface area (static charge) to mass (inertia). Thus, fines can cling by static electric forces to the electrically grounded metal wall of the reactor. Polymerization in the stagnant layer of reactor wall fines releases heat which can lead to melting and fusing of polymer into sheets along the vessel wall. These sheets of fused polymer may grow quite large before coming loose and falling into the fluid bed. Once fallen into the main fluid bed, such sheets can obstruct powder fluidization, circulation, and withdrawal. In some cases, the sheets may be so large as to significantly disrupt the normal fluidization and circulation of gas and solids of the entire fluid bed leading to extensive fusing of the main bed. When powder withdrawal slows or the bed fuses, the reactor production must be stopped and the reactor vessel opened for cleaning. This is a very costly production outage.
Another undesirable place where fines accumulate is in the disengaging section of the reaction vessel. The disengaging section of the reactor is a region of expanded cross-sectional area that is above the zone in which bed level normally resides. The purpose of the expanded area is to reduce the velocity of the fluidizing gas in order to minimize the entrainment of fine particles in the gas leaving the reactor. In the disengaging section, fines tend to concentrate in the regions of lower gas velocity nearer the downward sloping vessel wall. In fact, it is intended that most of the fines fall onto the sloped wall of the disengaging section and slide downwards and back into the main fluidized bed. However, when the concentration of fines increases in the disengaging section, the polymerization heat load on the sloped wall becomes larger. This can be observed by increasing temperatures seen from indicators placed in the sloped wall in the disengaging section. The concentration of fines and resulting heat can become great enough to lead to melting and fusing of the powder into sheets along the sloped wall. These sheets will tend to grow until their own weight and hydrodynamic forces cause them to fall into the main fluid bed, there, as discussed above, obstructing powder withdrawals and possibly causing more extensive bed fusing. As with the above, when either of these occur, the reactor production must be stopped and the reactor vessel opened for cleaning.
Several technologies are known for modification of static and of wall sheeting behavior. These include U.S. Pat. No. 4,803,251 demonstrating the addition of different compounds to the fluid bed reactor to adjust static voltage; U.S. Pat. No. 4,532,311 disclosing the use of chemicals to reduce the bonding of sheets to the reactor vessel wall; European Patent Application 0,453,116,A1 disclosing the addition of known antistatic agents to a fluid bed polyolefin reactor; European Patent Application 0,604,990,A1 disclosing the use of an electrode near the grid; and U.S. Pat. No. 5,461,123 disclosing the use of sound waves to dislodge fines from the reactor wall.
A particularly relevant discussion of the operating problems brought on by the presence of fines in the disengaging section of the reactor is contained in U.S. Pat. No. 5,428,118. This patent discloses the modification of the circulating pattern of fines in the disengaging section in order to reduce operating and product quality problems.
In addition to problems caused inside the reactor, problems are also caused by fines outside the fluid bed reactor vessel. Some fines will leave the reactor vessel in the overhead piping that carries the recycle gas away for cooling and compression. The exiting fines may attach to surfaces of piping, heat exchangers, and other process equipment in the recycle loop. Recycled fines may also settle in regions of lower gas velocity, such as the bottom of the reactor underneath the distribution grid for the fluidizing gas.
Because fines exiting the reactor retain their catalytic activity, they continue to react outside the reactor. Thus, fines depositing in the recycle system equipment grow and fuse to create skins, sheets, and lumps of polymer. These skins, sheets, and lumps reduce heat transfer efficiency and modify mass flow in the recycle gas piping and equipment. Also, some fines will return to the reactor via the recycle system. Because the temperature and gas composition are very different at some locations in the recycle system, the polymer produced outside the reactor may have very undesirable properties. Although a minute fraction of the total polymer production, the fines returning to the reactor can nonetheless seriously impact the suitability of the overall product. The presence of non-homogeneous polymer fines in the final product can significantly affect the quality of the product and resulting articles produced therefrom, such as the formation of gels in polyethylene films.
There are several disclosed methods for coping with reactive fines exiting a fluidized bed reactor. U.S. Pat. No. 4,882,400 discloses the use of cyclonic separators to remove the fines from gas exiting the reactor and the use of ejectors to return the fines to the reactor. U.S. Pat. No. 4,956,427 discloses technology for coating the surface of heat exchanger tubes to reduce the adhesion of fines and the formation of insulating polymer skins. U.S. Pat. No. 5,126,414; U.S. Pat. No. 4,933,149; and U.S. Pat. No. 5,352,749 all reveal improvements to mitigate problems with the accumulation of fines and lumps underneath the gas distribution grid in the reactor.
The above patents disclose methods of coping with the problems caused by polymeric fines only after they are produced. These references disclose various methods of preventing sheeting, through either physical, electrostatic, or chemical means of controlling the behavior of the fines. However, none of the above references discloses a method of preventing or reducing the production of fines. None of these references deal with the original problem of the formation of the fines during polymerization.
However, techniques for the modification of catalysts have been used for controlling the level of fines in the fluid bed. The size of the catalyst support, the size of the finished catalyst, the physical strength of the support and the chemical activities of the catalyst system are some of the catalyst parameters known to affect the particle size distribution in a fluid bed polyolefin reactor. Examples are given by P. Galli. in "The `Spherilene` Process", paper S4A, Polyethylene--The 1990s and Beyond, The Plastics and Rubber Institute (UK), (1992) and by I. D. Burdett in "A Continuing Success: The UNIPOL Process", P 616-623, CHEMTECH, October 1992. Unfortunately, when the catalyst is altered, then the process parameters and polymer properties are also altered. Catalyst changes often result in a modified polymer, such as changes in the density and molecular weight distribution.
In light of the above, it would be very desirable to be able to control to a low level or to reduce the amount of fines produced in the reactor, thereby avoiding the necessity of dealing with the problems caused by the fines, but without having to modify the catalyst, thereby avoiding changes to the properties of the resulting polyethylene polymer.